Aluminum products exhibit excellent properties such as lightweight, high workability and good heat conductivity, and aluminum products are thus widely used in the consumer-electronic industry and the motor vehicle industry, for example. The aluminum products are generally used after aluminum oxide coatings are formed on their surfaces by anodizing. The aluminum oxide coatings enhance surface hardness, corrosion resistance, abrasion resistance and colorability of the aluminum products.
An amount of time required in the anodizing of aluminum is desired to be shortened to improve productivity. In order to shorten the required time, generally, electric current density in the anodization is increased. The increase of the electric current density accelerates the forming of the anodic oxide coating, but may increase an amount of heat generation causing dissolution of the formed coating. Patent Document 1 (JP 2010-168642 A corresponding to US 2011/0203933 A1) suggests that conditions, such as ion conductivity in electrolysis solution or a surface temperature of an aluminum object, are controlled to accelerate a forming speed of anodic oxide coating in anodization of the aluminum object. As in Patent Document 1, speeding-up of anodizing of aluminum is desired.
When a tubular object made of aluminum or aluminum alloy is anodized, anodic oxide coating may become different in thickness between on an outer surface and an inner surface of the tubular object. In other words, the anodic oxide coating is more difficult to be thickened on the inner surface of the tubular object than on the outer surface of the tubular object. As a result, the thickness of the anodic oxide coating on the inner surface may become smaller than the thickness of the anodic oxide coating on the outer surface. This thickness difference tends to increase in accordance with a speed of forming of the anodic oxide coating, in other words, the thickness difference tends to increase in accordance with increase of the electric current density.